The present invention relates to a method of inspecting a quiescent power supply current in a semiconductor integrated circuit, and more particularly, to a method of determining a reference value for the quiescent power supply current.
As an example of an inspection method of the semiconductor integrated circuit, in particular of a CMOS circuit, a quiescent power supply current inspection is generally executed in which a defect determination is made on the basis of an ideal power supply current value for an internal circuit in a quiescent state thereof by utilizing the fact that scarcely little power supply current flows through the internal circuit in the quiescent state (see JP-A-2000-241492, for reference).
As an example of a known method of inspecting the quiescent power supply current, there is a method in which respective values of the quiescent power supply current is measured at each point of a plurality of nodes in the internal circuit to detect any abnormal quiescent power supply current value measured at each point, thereby making the defect determination on the basis of the detection. In this case, the reference value for the quiescent power supply current used for the defect determination is determined in consideration of discrepancy in the manufacturing process or on the basis of the respective values of the quiescent power supply current for defective and non-defective samples.
However, according to the method of inspecting the quiescent power supply current known in the art, since the reference value for the quiescent power supply current is determined in consideration of the discrepancy of the manufacturing process, it is necessary to loosely set the reference value for the quiescent power supply current used for the defect determination.
FIG. 13 is a diagram showing the distribution of the quiescent power supply current in the semiconductor integrated circuit. In FIG. 13, IddqA denotes a reference value for the quiescent power supply current determined in a conventional manner. The quiescent power supply current shows a number of distribution curves A, B, or C, due to the discrepancy in the manufacturing process, as shown in FIG. 13. Since a sample is determined to be defective only when its quiescent power supply current value exceeds the reference value IddqA, the samples corresponding to the portion (indicated by “X” in the figure) of the distribution curve B are also determined to be non-defective. However, the samples corresponding to the X portion should have been determined to be defective.
When the reference value is determined on the basis of the respective values of the quiescent power supply current for defective and non-defective samples in accordance with the known method, it is necessary to classify samples into the defective samples and the non-defective samples in advance.